Two new viologen-polyoxometalate hybrid crystalline materials: (MV)₂[HPW₂^ⅤW₁₀^ⅥO₄₀] ·2H₂O (1) and (EV)₂[Mo₈O₂₆] (2) have been synthesized by using the cations of 1, 1'-dimethyl-4, 4'-bipyridinium dichloride (methyl viologen, MV) and 1, 1'-diethyl-4, 4'-bipyridinium dibromide (ethyl viologen, EV) as the electron acceptors, and the electron-rich polyoxometalate anions as the electron donors. The structures of compounds 1 and 2 have been determined by single-crystal X-ray crystallography. Hydrogen bond interactions exist between the cations and the anions in 1 and 2. Interestingly, 2 has a photochromic performance with a light response time within 1 min. The photochromic mechanism of compound 2 has been investigated by solid-state diffuse reflection, electron paramagnetic resonance and theoretical calculation. 1 and 2 show good catalytic performance in the photocatalytic degradation of several organic dyes (methylene blue, pararosaniline hydrochloride and rhodamine 6G).

Two metal-organic frameworks (MOFs), trans-[Co(L)(μ₂-H₂O)(H₂O)₂]·2H₂O (1) and cis-[Mn(L)(Bipy)] (2) (H₂L=2, 2′-dimethyl-4, 4′-biphenyldicarboxylic acid, Bipy=4, 4′-bipyridine), have been synthesized and characterized by FTIR, thermogravimetric analysis (TGA), powder and single crystal X-ray diffraction. MOF 1 crystallizes in the triclinic system with a P1 space group and contains two crystallographically different Co(Ⅱ) ions. Each trans-[CoO₆] octahedron is connected by μ₂-H₂O and L^2- ligand with a bis(unidentate) coordination mode to produce a 2D sql topological network. MOF 2 crystallizes in the monoclinic system with a C2/c space group. The Mn(Ⅱ) cation adopts a cis-[MnO₄N₂] octahedron as a 6-connected node and is linked by L^2- ligand as a 4-connected node to generate a binodal (4, 6)-connected 3D fsc framework. The intermolecular interactions in 1 and 2 have been investigated by 3D Hirshfeld surface analyses and 2D fingerprint plots to reveal that the main interactions are H…H and O…H/H…O contacts in 1, and H…H and C…H/H…C contacts in 2. The TGA indicated that 1 and 2 were stable below 390 and 370 ℃, respectively.